The untreated exhaust gas of diesel engines contains, in addition to carbon monoxide CO, hydrocarbons HC and nitrogen oxides NOx, a relatively high oxygen content of up to 15% by volume. Furthermore, said untreated exhaust gas contains particle emissions which are composed predominantly of soot residues and possibly organic agglomerates and which originate from a partially incomplete combustion of fuel in the cylinder.
To adhere to the legal exhaust-gas limit values for diesel vehicles which will be applicable in future in Europe, North America and Japan, the simultaneous removal of particles and nitrogen oxides from the exhaust gas is necessary. The pollutant gases carbon monoxide and hydrocarbons can easily be made harmless from the lean exhaust gas by oxidation on a suitable oxidation catalytic converter. Diesel particle filters with and without additional catalytically active coatings are suitable devices for the removal of the particle emissions. The reduction of the nitrogen oxides to form nitrogen (“denitrogenization” of the exhaust gas) is more difficult on account of the high oxygen content. One known method is the selective catalytic reduction (SCR) of the nitrogen oxides on a suitable catalytic converter, or SCR catalytic converter for short. Said method is currently preferred for the denitrogenization of diesel engine exhaust gases. The reduction of the nitrogen oxides contained in the exhaust gas takes place in the SCR process with the aid of a reducing agent which is dosed into the exhaust section from an external source. As reducing agent, use is preferably made of ammonia or of a compound which releases ammonia, such as for example urea or ammonium carbamate. The ammonia, which is possibly generated in situ from the precursor compound, reacts on the SCR catalytic converter with the nitrogen oxides from the exhaust gas in a comproportionation reaction to form nitrogen and water.
At present, in order to satisfy the upcoming legal regulations, a combination of the different exhaust-gas purification units is inevitable. A device for the purification of diesel engine exhaust gases must comprise at least one oxidation-active catalytic converter and, for denitrogenization, an SCR catalytic converter with an upstream device for introducing reducing agent (preferably ammonia or urea solution) and an external reducing agent source (for example an auxiliary tank with urea solution or an ammonia store). If it is not possible, by optimizing the engine-internal combustion, to keep the particle emissions sufficiently low that they can be removed by the oxidation catalytic converter by means of direct oxidation with oxygen, the use of a particle filter is additionally necessary.
Corresponding exhaust-gas purification systems have already been described; some are presently at the stage of practical testing.
For example, EP-B-1 054 722 describes a system for the treatment of NOx and particle-containing diesel exhaust gases, in which an oxidation catalytic converter is connected upstream of a particle filter. A reducing agent source and a dosing device for the reducing agent, and also an SCR catalytic converter, are arranged at the outflow side of the particle filter.
US 2007/0044456 describes an exhaust-gas aftertreatment system which comprises, at the inflow side of a urea SCR catalytic converter (preferably transition metal/zeolite formulation with optimum NOx conversion in the temperature range between 200 and 500° C.), an oxidation catalytic converter (platinum-containing high-grade metal catalytic converter), and at the outflow side of the SCR catalytic converter, a diesel particle filter. A dosing device for urea is arranged between the oxidation catalytic converter and the SCR catalytic converter.
Both systems have in common that the untreated exhaust gas generated by the engine is conducted, in the first aftertreatment step, via an oxidation catalytic converter. The inventors have now established that such systems, which comprise an oxidation catalytic converter as the first exhaust-gas aftertreatment stage, are not suitable, without the inclusion of additional auxiliary measures, for purifying the exhaust gas of diesel engines of the most modern type, as are provided for example for EU-VI vehicles, to such an extent that the prescribed nitrogen oxide emission limit values can be adhered to.